The nearest expiration date for ULSD 10ppm CIF MED (Platts) vs. Low Sulphur Gasoil Futures (Jul 2017) is Aug 2, 2017.
The best time to sell a futures contract is when you've already made money on the investment, but still have plenty of time left before the expiration date. Thus, a good option would be to sell ULSD 10ppm CIF MED (Platts) vs. Low Sulphur Gasoil Futures (Jul 2017) before Aug 2, 2017.